On the Melting Point of Acetanilide Several years ago, students in our organic chemistry laboratory were synthesizingacetanilide by reacting aniline with acetic anhydride in acidic media. This is a standard synthetic procedure and is given in many laboratory manuals (1-3). After isolating and recrystallizing the product from water, some students found that the melting point was significantly lower than the literature value of 114'C. Indeed, several observed a narrow melting point range of about 83-84'C. Only after thoroughly drying the crystals were they able to duplicate the literature value. We find that the same problem has occurred a t Purdue University and a t the University of Illinois (where acetanilide is used asan unknown for freezing point depressions). Most major laboratory manuals make no mention of this phenomenun in their discussion of the synthesis and purification of acetanilide. Acetanilide and water form a binary system with only partial miscibility in the liquid phase.'The system has a quadruple point eonsistingaf two liquid phases, one solid phase, and the vapor. For the acetanilidelwater system, thequadruple temperature occurs a t 8 3 T . Below 83'C, acetanilide exists as a solid in the presence of any water clinging to the crystals. At 8 3 T , if water is present, a second liquid layer will begin to form consisting of liquid acetanilide containing dissolved water. The wet crystals may give the appearance of a sharp melting paint (83-84') if just the right amount of water happens to be present. Similar behavior is observed with a variety of substances that are frequently crystallized from water; benzoie acid and suecinonitrile are two examples. The changing melting points (wet versus dried) have led to many useful discussions in our organic laboratory sections. All binary systems in which organic solids may come out of solutions as an "oil," or second liquid phase, have quadruple points. These phenomena warrant a discussion of the appropriate phase diagram; a good discussion of these phenomena is given in two standard textbooks (5,6). Literature Cited Ill Adams. R., .lohnron, .l.R., and Wilcur. C. F.,.lr., "Lahoralury Exporimonts in Organic Chemi=try,"6th ed.. Macmillan. New Yurk. 1970. p. 300. 121 Ault. A., "Techniquesand Experiments for OrganicChemistry.l'Helhn~ukPress, Hoston, 1973, P. 220. I:,) Hsherts, R. M.,Gilbert, J. C., Rudewsld. L. R..and ~ iA. S.. "An~Intnductwn ~ t o Modern ~ Experimental ~ Onanic ~ Chemistry.'. ~ 2nd . ad.. Holt, Rinehan and Winrton. New York. 1974. p. :IJR. 141 Kirk. R. E., and Othmor. D. F.IEdii,irsl. "Encyclspedia ~,IChemicalTechnslopy.'. Vol. I , lntecrcience,Now Ywk, 1947, p. 49. 151 V s ~ e lA. , I.."A Textkwk 0 1 P ~ a ~ t iOrganic ~sl Chemistr.~,":lrd ed., Lungman. London, 1966. pp. 91-36.
Ronald S. Lenox Fred J. Hadley Wabash College Crawfordsville. IN 47983
460 / Journal of ChemicalEducation
